The present invention relates to magnetic recording and reproducing apparatus (hereinafter referred to as VTRs) and more particularly to a method of producing a tracking error signal when performing the tracking control by the use of four different pilot signals.
A tracking error signal producing method which has heretofore been used will now be described.
FIG. 1 shows a magnetization pattern of four different recorded pilot signals. In the Figure, symbols A.sub.1, B.sub.1, A.sub.2, --, --designate recording tracks recorded by A and B heads of different azimuth angles, and f.sub.1 to f.sub.4 pilot signals. The frequencies of the pilot signals are selected to have values of 6.5 f.sub.H to 10.5 f.sub.H as shown in the Figure if f.sub.H represents the frequency of the horizontal synchronizing signal in the video signal. The pilot signals are sequentially and cyclically switched on every field so as to be superposed on information signals for recording. With the pilot signals recorded on the respective tracks, the frequency difference between the tracks is either f.sub.H or 3f.sub.H. Thus, by using the method which will be described later, it is possible to extract the frequency components f.sub.H and 3f.sub.H and compare their levels thereby utilizing the resulting signal as a tracking error signal.
In FIG. 1, numeral 101 designates the magnetic head, 102 the scanning direction of the magnetic head, and numeral designates 103 the direction of movement of the magnetic tape.
FIG. 2 is a block diagram showing a processing circuit for producing a tracking error signal. In the Figure, the reproduced pilot signal is applied through a terminal 201. For example, when the scanning position of the head is the one shown in FIG. 1, the reproduced pilot signal is a composite signal of f.sub.2, f.sub.3 and f.sub.4. A balanced modulation circuit 202 multiplies a reference signal introduced through a terminal 203 and the reproduced pilot signal. The reference signal has the same frequency component as the pilot signal recorded on the main track scanned by the head and it has the frequency f.sub.3 in the case of the head scanning shown in FIG. 1. The output signals of the balanced modulation circuit 202 are the sum and difference signals of the reference signal and the reproduced pilot signal and the difference signal is extracted by an f.sub.H tuning circuit 204 and a 3f.sub.H tuning circuit 205. Numerals 206 and 207 designate detector and rectifier circuits and numeral 208 designates a level comparison circuit. The output level of the level comparison circuit 208 varies depending on the difference in level between the f.sub.H and 3f.sub.H signals and thus it can be used as a tracking error signal. Numeral 209 designates an analog inverter circuit and numeral 210 designates an analog switch. In response to a head switching signal (hereinafter referred to as an H.SW signal) applied through a terminal 211, the analog switch 210 is switched so as to generate inverted and uninverted signals. Here, the H.SW signal is a rectangular signal having a frame period (30 Hz in the NTSC system) and synchronized with the rotation phase of the magnetic heads. The reason for alternately generating the inverted and uninverted signals for every field is to cause the track deviation direction of the magnetic head and the varying direction of the tracking error signal to always coincide with each other. For example, there is a difference in the varying direction of the f.sub.H and 3f.sub.H signals with respect to the deviation of the head in the same direction between the case where the head scans an Ai track (i=1, 2, 3, --, --) and the case where the head scans a Bi track and therefore the polarity of the signal must be reversed every field.
The tracking error signal generated at a terminal 212 is supplied to a capstan control system. The capstan control system utilizes the tracking error signal to control the driving phase of the magnetic tape in such a manner that the magnetic head follows and scans the recording track for reproduction.
The waveforms generated at various parts in the circuit block diagram shown in FIG. 2 will now be described.
FIG. 3 shows the relative positions of A magnetic head 301 and B magnetic head 302 and the recording tracks when the former are following or on-track and scanning the latter for reproduction and FIG. 4 shows the resulting signal waveforms.
Shown in (a) of FIG. 4 is the H.SW signal and A.sub.1, B.sub.1, --, --show time periods in which the tracks A.sub.1, B.sub.1, --, --shown in FIG. 3 are respectively scanned for reproduction. Shown in (b) of FIG. 4 is the output signal of the f.sub.H tuning circuit 204 and (c) the output signal of the 3f.sub.H tuning circuit 205. In the on-track condition, the reproduced levels of the f.sub.H and 3f.sub.H signals are the same. Shown in (d) is the output level of the level comparison circuit 208. In the on-track condition, this output level has for example a value of 1/2 Vcc (Vcc is the supply voltage). Shown in (e) is the tracking error signal generated at the terminal 212. The inverting circuit 209 inverts the level variation with respect to the 1/2 Vcc and therefore the signals (d) and (e) become equal to each other in the on-track condition.
FIG. 5 shows a condition in which the heads are deviated from the respective tracks to the left on the paper plane and FIG. 6 shows the resulting waveforms at the various parts. As shown in the Figure, the f.sub.H signal component is smaller than the 3f.sub.H signal component during the scanning period of the A head and the reverse relation takes place during the scanning period of the B head. As a result, the output (d) of the level comparison circuit has a rectangular waveform on both sides of 1/2 Vcc as shown in the Figure. Here, the waveform is drawn on the assumption that the output of the level comparison circuit goes high when the level of f.sub.H is greater than the level of 3f.sub.H. If the polarity of the output of the level comparison circuit generated during the scanning periods of the B head is inverted with respect to the 1/2 Vcc, the resulting tracking error signal becomes as shown in (e) of FIG. 6 and a level variation 601 from the 1/2 Vcc corresponds to the track deviation.
Next, a description will be made of the waveforms generated at the various parts when the A head and the B head are different in height.
The head heights of the A and B heads are adjusted so as to become the same from the reference plane perpendicular to the rotation axes. In fact, however, there occurs an adjustment error. This head height adjustment error is equivalent to the fact that one head relatively deviates in the width direction of the recording track on the basis of the other head. FIG. 7 shows the relative positional relation obtained when the normal recording tracks recorded by the heads involving no head height difference are reproduced by the heads involving such head height difference. With the heads involving the head height difference, the control system is stabilized with the head positions shown in FIG. 7 as will be described later in detail.
FIG. 8 shows the respective signals generated in response to the head positions shown in FIG. 7. In FIG. 8, the solid lines show the signals generated in response to the head positions shown in FIG. 7 and the broken lines show the signals generated in response to the heads deviated in the directions of arrows 701 and 702, respectively, from the head positions shown in FIG. 7.
The signals shown by the solid lines will be described first.
With the head positions shown in FIG. 7, the signal including the f.sub.H frequency component is always at the low level and the 3f.sub.H component is always at the high level during the scanning periods of the A and B heads. As a result, the output of the level comparison circuit varies as shown by the solid line in (d) of FIG. 8 and the tracking error signal inverted during the scanning periods of the B head becomes as shown by the solid line in (e) of FIG. 8. The tracking error signal is sent to the capstan control system through a low-pass filter so that its average level becomes 1/2 Vcc and the control system is stabilized in this condition. Thus, it can be said that the head positions shown in FIG. 7 represent stable positions.
Next, a description will be made of the signals generated in response to the deviation of the heads of FIG. 7 in the directions of the arrows 701 and 702. In this case, the signals become as shown by the broken lines in FIG. 8. In other words, the f.sub.H component signal is decreased further and conversely the 3f.sub.H component signal is increased during the scanning periods of the A head. During the scanning periods of the B head, conversely the f.sub.H component signal is increased and the 3f.sub.H component signal is decreased. Thus, the output signal of the level comparison circuit becomes as shown by the broken line in (d) of FIG. 8. On the other hand, the tracking error signal obtained by inverting the polarity of the signal with respect to the 1/2 Vcc level during the scanning periods of the B head becomes as shown by the solid line in (e) of FIG. 8.
As will be seen from the foregoing description, the voltage variation corresponding to the track deviations takes the form of a level variation 801 between the signals generated during the scanning periods of the heads in the case of the output of the level comparison circuit, while in the case of the tracking error signal, it takes the form of a differential voltage 802 between the average DC potential of the tracking error signal level variations and the 1/2 Vcc potential. On the other hand, the voltage variation due to the head height difference is such that it appears as a differential voltage 803 between the average DC potential of the output signal and the 1/2 Vcc potential in the case of the output of the level comparison circuit and it appears as a tracking error signal level variation 804 in the case of the tracking error signal.
The level variation 804 shown in (e) of FIG. 8 is not completely removed by its passage through the low-pass filter and there remains a variation component of the H.SW period. This results in a variation in the rotation speed of the capstan motor thus causing problems including a picture flagging, etc.